1. Ph.D Theses

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Author: search.filters.author.Joladarashi, SharnappaHas files: Yes

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    Performance Evaluation of Sandwich Composites with Functionally Graded Core for Ballistic Impact
    (National Institute of Technology Karnataka, Surathkal., 2024) T.S., Mohan Kumar; Joladarashi, Sharnappa; S.M., Kulkarni
    Sandwich composites with flexible skin and stiff core are appropriate for a wide range of engineering applications because of their capacity to withstand greater deformation while maintaining a high load-carrying capacity. The main objective of the current research includes material selection, fabricating, and analyzing functionally graded sandwich composite for ballistic impact applications. Statistical Six Sigma DMAIC methodology was used for material selection, incorporating qualitative and quantitative approaches, ensuring a comprehensive and accurate material selection process. Considering a careful literature review, the choice of jute and rubber for skin material paired with epoxy and sea sand for core material in the sandwich composite. Finite element (FE) studies, based on the rule of mixtures, estimated composite material properties, showing increased energy absorption and a decrease in residual velocity with higher filler composition (0% to 30%) at velocities of 10, 50, 100, and 350 m/s, and with core thickness from 10mm to 30mm at 350 m/s. Following the initial FE studies, experimental testing was conducted on composite coupons to evaluate their physical and mechanical properties. The gradation test was performed to check the functional gradience for the core material. These tests provided insights into sea sand's spatial distribution and gradation within the core samples, emphasizing the impact of sea sand composition on the stepwise layering gradation. The void % (3.44%) in the composite coupons increases as the filler composition increases. Specific tensile strength decreases (2.41 times) with an increase in the filler composition. Hardness (12.47%), Flexural strength (27.93%), and impact strength (2.35 times) increase as the filler composition increases compared to neat epoxy. High strain rate compression strength is improved with higher strain rates. The FE analysis for low and ballistic impact testing was conducted based on the properties obtained experimentally from the composite coupons. For low-velocity and impact testing, it is observed that a sandwich with 30% sea sand composition has superior damage resistance capabilities compared to its counterparts. Both experimental and FE analyses show that higher filler percentages lead to increased energy absorption. The sandwich composite with 30% sea sand exhibited the lowest depth of damage and minimized overall damage across all impact energies, demonstrating superior damage resistance compared to other compositions. For ballistic impact, the similar trend that increasing the volume percentage of sea sand and core thickness improved energy absorption, with a 30 vol% sand content and 30mm core thickness performing the highest energy absorption capability compared to its counterparts. The results indicate that while thinner cores (10 mm) are inadequate for arresting projectiles, thicker cores (20 mm and 30 mm) show progressively better performance, with the 30 mm core providing the most effective ballistic impact protection. Incorporating sea sand into epoxy reduced costs by 8.9% to 32.47%, making it an economical core material for ballistic impact applications. Experimental and FE simulations for the entry area at 200 m/s showed a damage area percentage error of about 12.61%. Fractography analysis indicated face sheet damage from compression and bending, fiber breakage, rubber tearing, and core failure from sand particle crushing and matrix cracking, with a river like pattern suggesting brittle failure.
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    Elevated Temperature Sliding Wear Behavior Of Cocrnitimox And Cocrnitiwx High Entropy Alloys Processed Using Mechanical Alloying and High-Velocity Oxy-Fuel Spray
    (National Institute Of Technology Karnataka, Surathkal., 2024) Addepalli, Syam Narayana; Joladarashi, Sharnappa; M.R., Ramesh
    Maraging steels, widely used in the aircraft landing gear components were subjected to wear due to the harsh working conditions. Surface modification of these components by the deposition of advanced coating materials prolong their life. High entropy alloys (HEA) are a contemporary class of materials with multiple primary elements having applications in different fields, owing to their exceptional mechanical and physical properties. Therefore the curent research is aimed at enhancing the wear performance of maraging steels, by the deposition of HEA coatings. CoCrNiTiMox and CoCrNiTiWx (x: molar ratio; x= 0.5, 1, 1.5) HEAs were processed by mechanical alloying of pure metal powders for further application as feedstock in the High velocity oxy-fuel (HVOF) technique. The phase and microstructural transformation of the ball milled powders is investigated in detail by optimizing the milling time and speeds. The milling process is extended for 50 h and milled powder samples were collected at regular intervals of 10, 20, 30, 40 and 50 h to characterize the samples for their suitability to deposit using thermal spray techniques. The milled powders were characterized with respect to the phases, particle morphology, chemical homogeneity, particle size and crystallite sizes. Based on the characterization studies, the powders milled at a speed of 200 rpm for 10 h were selected as feedstock for HVOF deposition. After the deposition of coatings, the microstructural and mechanical characterization of coatings were performed. The phases and microstructure of the deposited HEA coatings were determined by X-ray diffraction (XRD) and scanning electron microscope (SEM). The microhardness of the coating was determined by using a vickers indenter on the coatings cross-section, with a load of 300 g and a dwell time of 15 s. The deposited coatings fracture toughness was determined by using the Evans and Wilshaw’s approach. The tribological behaviour of CoCrNiTiMox and CoCrNiTiWx HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiMox and CoCrNiTiWx HEA coatings, as calculated using ImageJ software, were found to be in the range of 1-2%. i The mechanical performance of the CoCrNiTiMox and CoCrNiTiWx HEA coatings revealed superior values, when compared to other HEA coatings. The microhardness of CoCrNiTiMo0.5, CoCrNiTiMo, and CoCrNiTiMo1.5 HEA coatings were found to be 841±62 HV0.3, 927 ± 45 HV0.3 and 952±23 HV0.3, respectively. On the other hand, the microhardness of CoCrNiTiW0.5, CoCrNiTiW, and CoCrNiTiW1.5 HEA coatings were found to be 863±52 HV0.3, 951 ± 38 HV0.3 and 1025±39 HV0.3, respectively. The fracture toughness of CoCrNiTiMo0.5, CoCrNiTiMo, and CoCrNiTiMo1.5 HEA coatings were found to be 2.89 ± 0.31 (Mpa m1/2), 3.26 ± 0.25 (Mpa m1/2) and 3.79 ± 0.35 (Mpa m1/2) respectively. Likewise, the fracture toughness of CoCrNiTiW0.5, CoCrNiTiW, and CoCrNiTiW1.5 HEA coatings, were found to be 3.22 ± 0.26 (Mpa m1/2), 3.54 ± 0.32 (Mpa m1/2) and 3.87 ± 0.3 (Mpa m1/2) respectively. Further, it can be witnessed that the as-sprayed HEA coatings exhibited a steady increment in the mechanical properties with an increment in the molar fraction of Molybdenum and Tungsten. The specific wear rate of CoCrNiTiMo HEA coating dropped by 70.5%, declining from 17.34 ± 2.8 x10-6 mm3/N-m to 5.1 ± 1.6 x10-6 mm3/N-m, while CoCrNiTiW dropped by 76.3%, decreasing from 15.8 ± 3.7 x10-6 mm3/N-m to 3.73 ± 2.1 x10-6 mm3/N-m, with an increase in the temperature from RT to 600 °C. The wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, CoMoO4, NiO tribofilms for CoCrNiTiMo, and TiO2, CoWO4, WO3 oxides for CoCrNiTiW. Further, the CoCrNiTiMo1.5 HEA coatings offered better wear resistance, as compared to CoCrNiTiMo0.5 HEA coatings, at any temperature and loading condition, due to the increment in the molar fraction of Molybdenum. Additionally, the CoCrNiTiW1.5 HEA coatings exhibited superior wear performance, when compared to all the six compositions in the current research. The investigation of worn surfaces showed a transformation in wear mechanisms from adhesive and abrasive wear at room temperature to oxidative wear at elevated temperatures.